1. Field of the Invention
The present invention relates to a pulse sequence for use in operating a nuclear magnetic resonance tomography apparatus, and in particular to such a pulse sequence for operating the apparatus to produce images with different T.sub.2 contrast.
2. Description of the Prior Art
The contrast of tomograms produced in nuclear magnetic resonance tomography depending upon the relaxation times T.sub.1 and T.sub.2, and on the density of the substance under examination. Because the relaxation time T.sub.2 is defined, inter alia, by the chemical bond of the nuclides under observation, it is of use in providing information regarding the examination subject. Images having different informational content can be produced on the basis of different T.sub.2 contrast, i.e., different brightness of a picture element dependent on the corresponding T.sub.2 time.
A multi-echo sequence wherein images which are T.sub.2 -weighted to different degrees are obtained is described in "Multiple Spin Echo Imaging With A 2d Fourier Method," Graumann et al., Magnetic Resonance in Medicine, Vol. 3, pages 707-721 (1986). The multi-echo in this method is achieved using a spin echo with at least two refocusing pulses.
A method for sodium imaging is described in "Sodium Magnetic Resonance Imaging of Human Body," Ra et al., SMRM (1986), Book of Abstracts, pages 1462-1463. A problem with this type of imaging is that sodium has extremely short T.sub.2 components. To acquire these short T.sub.2 components, the FID signal is first directly evaluated after a 90.degree. pulse, and the topical resolution is undertaken using a back-projection method. After a spin echo produced by a 180.degree. RF pulse, a signal is again read out, by which signal components having longer T.sub.2 times are acquired. The same back-projection method is then used again.